Generally, a nitride semiconductor layer is formed on an inexpensive silicon substrate or sapphire substrate. However, lattice constants of these substrates are greatly different from that of the nitride semiconductor layer, and thermal expansion coefficients are also different therebetween. Thus, considerable strain energy is produced in the nitride semiconductor layer formed on the substrate by epitaxial growth. As a result, the nitride semiconductor layer tends to suffer from crack formation or a reduction in crystal quality.
To solve the problem, as a conventional practice, a buffer layer in which nitride semiconductor layers with different compositions are stacked is arranged between a substrate and a functional layer made of a nitride semiconductor.
Moreover, as disclosed in Patent Documents 1 to 3 and so forth, a GaN-on-Si-based semiconductor is provided with composition graded layers between layers in a GaN-based multilayer buffer layer so as to control stress, suppress cracks, and improve crystallinity.